The present disclosure relates to semiconductor integrated circuit devices and, more particularly, to a flash memory device and a smart card including the same.
Semiconductor memory devices are generally classified into volatile or nonvolatile depending on whether memory data can be retained or not in the absence of a power supply. Random access memories, such as SRAM and DRAM, are typically volatile memory devices. Nonvolatile memory devices include various kinds of read-only memories (ROMs), for example, erasable and programmable ROMs (EPROMs), electrically erasable and programmable ROMs (EEPROMs), and flash memories.
Recently, flash memory devices are becoming highly interesting to designers in view of their merits of small size, low power consumption, and advanced performance of reading/writing. For example, flash memory devices are usually employed in providing an on-chip memory system for portable apparatuses, such as cellular phones, digital cameras, audio/video recorders, modems, smart cards, and so forth. Those portable apparatuses are required to contain information that needs a fast data update.
A flash memory cell, for example, a split-gate flash memory cell is operable with the mechanism of F-N tunneling for erasing data of the memory cell or source-side channel hot electron injection for programming data therein. To accomplish the source-side channel hot electron injection for a memory cell to be programmed, a word line of the selected memory cell is driven with a voltage of about 1.2 V and a source line of the selected memory cell is driven with a voltage of about 9V. For data to be programmed (that is, program data), a bit line of the selected memory cell may be driven with a voltage of about 0.3V. According to this bias condition, a current flows from the bit line to the source line through the selected memory cell, which means there is an actual current consumption. On the other hand, for data to be inhibited against programming (that is, program-inhibited data), a bit line of a selected memory cell may be driven by a power source voltage, which makes the selected memory cell turned off to interrupt a current toward the bit line from the source line.
According to a previously known method of programming, an amount of current dissipated during the programming operation may be variable based on the number of data bits to be programmed. This means a voltage of the source line becomes lower in view of the dissipated amount of current during the programming operation. As a result, such a voltage drop on the source line may cause a degradation in program characteristics of the memory cells.
Therefore, a new technique is needed to maintain a rate of current consumption at a constant level, regardless of the number of data bits to be programmed.